Though the material has the same crystal structure--cubic, like diamond and spinel--garnets are an entire mineral group that vary in their chemical composition, resulting in a variety of gems in different colors and with different qualities. Though some varieties of red garnet are common and found on every continent on Earth, other garnets like orange spessartite and hessonite, and green demantoid and tsavorite, are much less abundant. There are more than 20 garnet species, but the five most important gem garnet species include pyrope and almandite (the combination of which creates rhodolite), spessartite, grossular or grossularite (which includes hessonite and tsavorite) and andradite (which includes demantoid).
Some garnets exhibit phenomena that other garnets do not. Almandite and sometimes rhodolite garnets can exhibit asterism, displaying rare four-ray or even rarer six-ray stars. The andradite species can display chatoyancy (cat's-eye) or iridescence, though both are very rare. Pyrope, or pyrope-spessartite mixes like Malaya, and select other garnets can possess color-change abilities. The grossular garnets exhibit no phenomena - at least none that have been discovered yet.
The unique range of properties, appearance and history of the garnets make them a popular gem to collect. Ancient texts and artifacts feature garnet. Red has long been one of the most prized of colors, signifying power and prestige. Garnet has probably been known to mankind longer than ruby. There are certain types of granite used in countertops that contain garnet crystals – look for the small, translucent reddish areas.